What is OpenMP?

What's a high-level description of OpenMP?

The Wikipedia article states that "OpenMP (Open Multi-Processing) is an application programming interface (API) that supports multi-platform shared memory multiprocessing programming in C, C++ and Fortran on many architectures, including Unix and Microsoft Windows platforms. It consists of a set of compiler directives, library routines, and environment variables that influence run-time behavior." What?

How does it compare to other approaches to concurrency, like threads, thread-pools, and work-stealing?